Optical transmitter input resistance measurement and encoder/driver modulation current configuration techniques

ABSTRACT

Techniques for automatically determining an input resistance of an optical modulator and configuring a modulation current source can include applying a first bias current to an input of the optical transmitter and measuring a corresponding first voltage at the input of the optical transmitter. A second bias current can also be applied to the input of the optical transmitter and a corresponding second voltage at the input of the optical transmitter can be measured. An input resistance of the specific optical transmitter can be determined from the difference between the first and second voltages divided by the difference between the first and second bias currents. The technique can further include setting one or more configuration settings in one or more registers of a modulation current source based on the determined input resistance of the optical transmitter. Thereafter, the output modulation current for driving the specific optical transmitter can be configured based on the one or more configuration settings in the one or more registers.

BACKGROUND OF THE INVENTION

Transmitter circuits are utilized to transmit data in a number ofapplications. Referring to FIG. 1, an exemplary transmitter circuit,according to the conventional art, is illustrated. The transmittercircuit 100 can include an encoder/driver 110 coupled to an opticaltransmitter 120. The encoder/driver 110 can be configured to convertthree signals at its inputs (In0-In2) into a pulse amplitude modulatedelectrical signal at its output (Out). In one implementation, theencoder/driver 110 can be a “PAM-4” encoder/driver 110 that utilizesfour amplitude levels of pulse modulation. In one implementation, PAM-4modulation on a 53.125 giga baud (GBaud) communication link can supportabout 100 giga bits per second (Gbps).

The optical transmitter 120 can be configured to convert an electricalsignal at its input (In) into an optical signal at its output (Out). Inone implementation, the optical transmitter 120 can be a vertical-cavitysurface-emitting laser (VCSEL) transmitter 120 that can convert thepulse amplitude modulated electrical signal from the PAM-4encoder/driver 110 into a PAM-4 modulated optical signal at its output.

A number of parameters, including the input resistance of the VCSELtransmitter 120, impact the performance of the PAM-4 encoder/driver 110and or the VCSEL transmitter 120 when coupled together. In aconventional implementation, one or more settings in the PAM-4encoder/driver 110 can be set to account for the input resistance of theVCSEL transmitter 120. However, the input resistance of the VCSELtransmitter 120 can be different for different power levels, can varyfrom manufacturer to manufacturer, can vary from unit to unit, and orthe like, which can make configuring the settings of the PAM-4encoder/driver 110 complicated. Therefore, there is a continuing needfor improved transmitter circuits and methods of configuring thetransmitter circuits.

SUMMARY OF THE INVENTION

The present technology may best be understood by referring to thefollowing description and accompanying drawings that are used toillustrate embodiments of the present technology directed toward opticaltransmitter input resistance measurement apparatus and methods andencoder/driver modulation current configuration apparatus and methods.

In one embodiment, a driver or encoder/driver can include a bias currentgenerator, an analog-to-digital converter and control logic. The controllogic can be configured to control the bias current generator to apply afirst bias current to an input of an optical transmitter and control theanalog-to-digital converter to measure a corresponding first voltage atthe input of the optical transmitter. The control logic can also beconfigured to control the bias current generator to apply a second biascurrent to the input of the optical transmitter and control theanalog-to-digital converter to measure a corresponding second voltage atthe input of the optical transmitter. The control logic can also beconfigured to determine an input resistance of the individual opticaltransmitter from a difference between the second and first voltagesdivided by a difference between the second and first bias currents.

The driver or encoder/driver can also include a modulation currentsource. The control logic can further be configured to set one or moreconfiguration settings of the modulation current source based on thedetermined input resistance of the specific optical transmitter.

In another embodiment, a method of configuring a driver orencoder/driver can include applying a first bias current to an input ofthe optical transmitter and measuring a corresponding first voltage atthe input of the optical transmitter in response to receiving a startupevent. A second bias current can also be applied to the input of theoptical transmitter and a corresponding second voltage can be measuredat the input of the optical transmitter in response to the receivedstartup event. An input resistance of the specific optical transmittercan be determined from the difference between the first and secondvoltages divided by the difference between the first and second biascurrents in response to the received startup event.

In another embodiment, a method of configuring a driver orencoder/driver can include determining automatically by theencoder/driver an input resistance of an individual optical transmitter.One or more configuration settings in one or more registers of amodulation current source can automatically be set by the driver orencoder/driver based on the received input resistance of the opticaltransmitter.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present technology are illustrated by way of exampleand not by way of limitation, in the figures of the accompanyingdrawings and in which like reference numerals refer to similar elementsand in which:

FIG. 1 shows an exemplary transmitter circuit, according to theconventional art.

FIG. 2 shows a portion of an encoder/driver coupled to an opticaltransmitter, in accordance with aspects of the present technology.

FIG. 3 shows a current feedback portion of the encoder/driver, inaccordance with aspects of the present technology.

FIG. 4 shows an encoder/driver and optical transmitter, in accordancewith aspects of the present technology.

FIG. 5 shows a method of configuring an encoder/driver for an individualunit of an optical transmitter, in accordance with aspects of thepresent technology.

FIG. 6 shows a bias current versus input voltage of an exemplaryvertical-cavity surface emitting laser, in accordance with aspects ofthe present technology.

FIG. 7 shows a method of configuring an encoder/driver for an individualunit of an optical transmitter, in accordance with aspects of thepresent technology.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the embodiments of the presenttechnology, examples of which are illustrated in the accompanyingdrawings. While the present technology will be described in conjunctionwith these embodiments, it will be understood that they are not intendedto limit the technology to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications, andequivalents, which may be included within the scope of the invention asdefined by the appended claims. Furthermore, in the following detaileddescription of the present technology, numerous specific details are setforth in order to provide a thorough understanding of the presenttechnology. However, it is understood that the present technology may bepracticed without these specific details. In other instances, well-knownmethods, procedures, components, and circuits have not been described indetail as not to unnecessarily obscure aspects of the presenttechnology.

Some embodiments of the present technology which follow are presented interms of routines, modules, logic blocks, and other symbolicrepresentations of operations on data within one or more electronicdevices. The descriptions and representations are the means used bythose skilled in the art to most effectively convey the substance oftheir work to others skilled in the art. A routine, module, logic blockand/or the like, is herein, and generally, conceived to be aself-consistent sequence of processes or instructions leading to adesired result. The processes are those including physical manipulationsof physical quantities. Usually, though not necessarily, these physicalmanipulations take the form of electric or magnetic signals capable ofbeing stored, transferred, compared, and otherwise manipulated in anelectronic device. For reasons of convenience, and with reference tocommon usage, these signals are referred to as data, bits, values,elements, symbols, characters, terms, numbers, strings, and/or the likewith reference to embodiments of the present technology.

It should be borne in mind, however, that these terms are to beinterpreted as referencing physical manipulations and quantities and aremerely convenient labels and are to be interpreted further in view ofterms commonly used in the art. Unless specifically stated otherwise asapparent from the following discussion, it is understood that throughdiscussions of the present technology, discussions utilizing the termssuch as “receiving,” and/or the like, refer to the actions and processesof an electronic device such as an electronic computing device thatmanipulates and transforms data. The data is represented as physical(e.g., electronic) quantities within the electronic device's logiccircuits, registers, memories and/or the like, and is transformed intoother data similarly represented as physical quantities within theelectronic device.

In this application, the use of the disjunctive is intended to includethe conjunctive. The use of definite or indefinite articles is notintended to indicate cardinality. In particular, a reference to “the”object or “a” object is intended to denote also one of a possibleplurality of such objects. The use of the terms “comprises,”“comprising,” “includes,” “including” and the like specify the presenceof stated elements, but do not preclude the presence or addition of oneor more other elements and or groups thereof. It is also to beunderstood that although the terms first, second, etc. may be usedherein to describe various elements, such elements should not be limitedby these terms. These terms are used herein to distinguish one elementfrom another. For example, a first element could be termed a secondelement, and similarly a second element could be termed a first element,without departing from the scope of embodiments. It is also to beunderstood that when an element is referred to as being “coupled” toanother element, it may be directly or indirectly connected to the otherelement, or an intervening element may be present. In contrast, when anelement is referred to as being “directly connected” to another element,there are not intervening elements present. It is also to be understoodthat the term “and or” includes any and all combinations of one or moreof the associated elements. It is also to be understood that thephraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting.

Referring now to FIG. 2, a portion of an encoder/driver coupled to anoptical transmitter, in accordance with aspects of the presenttechnology, is shown. The encoder/driver can include a plurality of“lanes” of modulators. For example, a PAM-4 encoder/driver can include a“top lane” modulator 210, a “middle lane” modulator 212, and a “bottomlane” modulator 214. The circuitry of each transmit lane 210-214 can besubstantially the same. Various parameters, including but not limitedto, delays, drive strength, data input and or the like of theencoder/driver can vary from transmit lane to transmit lane. Current,corresponding to input data, from the plurality of transmit lanes210-214 can be combined or summed in resistive element 230 to drive theoptical transmitter 220. The optical transmitter 220 can be, but is notlimited to, vertical-cavity surface emitting laser (VCSEL) transmitter220. The VCSEL transiter 220 can generate a light signal thatcorresponds generally in magnitude to the currents summed by theplurality of transmit lanes 210-214.

The encoder/driver combines a plurality of streams of data, for examplenon-return-to-zero (NRZ) data into one linear PAM-4 stream, and in sodoing the encoder/driver provides an ability to control and program biascurrent and modulation current, as well as provide the controlsnecessary to accommodate VCSEL characteristics in order to provide aclean optical eye of the modulation signal. Optical eye shaping of themodulation signal may be facilitated by per-lane programmable controlsfor pre and post de-emphasis. The post de-emphasis can include apolarity switch to allow both under and over equalization.Over-equalization ability is sometimes necessary when driving a VCSEL,as the characteristics of the VCSEL can be such that there isundesirable peaking on the rising data edge. In order to further helpthe post de-emphasis, on both rising and falling edges, a capacitor (notshown) can be placed on the emitter of a differential pair of a finaloutput stage of each data stream. This helps to slow the rising edge ofcurrent into the VCSEL, while helping to speed up the falling edge,thereby countering some of the tendencies of the VCSEL in the electricalto optical conversion. Additional controls allow the skew between thethree data streams to be programmed independently allowing furthercrucial eye shaping of the optical output.

A regulator 225 of the encoder/driver can be set to provide an averagebias current through a resistive element 230 to the VCSEL 220. Theregulator 225 can also support a transient current to generate themodulation current through the VCSEL 220. In one implementation theregulator 225 can be a low dropout (LDO) regulator. A plurality ofregisters can be provided in the encoder/driver to aid in setting a biascurrent and/or modulation current as will be further described below.

Top lane 210 can accesses a first data stream input (DataIn0). The datastream input can comprise binary, high/low data. The data stream inputcan be characterized as a pseudo-random bit stream (PRBS), in someembodiments. A loopback input (Loop_Bk Input) can be used in a test modeto run known data through top lane 210, via data checker signal(Data_Checker), to test data integrity, in some embodiments. In aworking, not-test, environment, the data stream input (DataIn0) can flowthrough a buffer 235 and a multiplexor 240 to a polarity inverter 245.Polarity inverter 245 can enable the data stream input (DataIn0) to beselectively inverted, if desired. A delay unit 250 can enable the datastream to be delayed, for example, by zero to about 0.25 unit interval(UI) (e.g., fraction of bit period) or more. Such a delay may beutilized to account for differing rise and fall times in a modulator,e.g., a VCSEL, according to current magnitudes, in embodiments. Forexample, a VCSEL may have a different rise time for a “1” bit in a topchannel in comparison to a “1” bit in a bottom channel.

The output of delay unit 250 can be sent to “pre” current source 255.Pre current source 255 can provide an early current corresponding to thedata stream input (DataIn0). The output of delay unit 250 can also besent to a duty cycle distortion correction unit 260. The duty cycledistortion correction unit 260 can function to selectively correct foroffsets or distortions in bit shapes in a data stream, e.g., due toprocess variations. In some implementations, the input to the precurrent source 255 can be coupled to the output of duty cycle distortioncorrection block 260 rather than the output of the delay unit 250. Theoutput of duty cycle distortion correction unit 260 can move through adelay stage 265 to a main current source 270. The main current source270 can provide an on-time current corresponding to the data streaminput (DataIn0). The output of duty cycle distortion correction unit 260can also move through multiple delay stages 275-280 to a post currentsource 285. The post current source 285 can provide a late currentcorresponding to the data stream input (DataIn0). Similarly, the middlelane 212 can shape a middle eye optical pattern, and the bottom lane 214can share a bottom eye optical pattern. The combination or sum of precurrent source 255, main current source 270, and post current source 285can be combined to form a desired optical wave shape for the top lane(or top “eye”) 210 for output to the VCSEL 220. In some embodiments, allsuch current sources may be programmable. Again, the circuitry of eachtransmit lane 210-214 can be substantially the same.

Referring now to FIG. 3, a current feedback portion of theencoder/driver, in accordance with aspects of the present technology, isshown. The circuitry within the dashed boundary represents a currentsource of a transmit layer, such as the main current source 270. Thecircuitry outside the dashed boundary represents a single currentfeedback loop. An exemplary PAM-4 encoder/driver can include ninecurrent sources, including a pre current source 255, a main currentsource 270 and a post current source 285 for the top lane 210, themiddle lane 212 and the bottom lane 214. The nine current sources can becoupled to a single current feedback loop.

Bias and modulation current levels are controlled by a feedback loop 300comprising a low drop out (LDO) regulator 305 and an operationalamplifier 310. Each output stage, for example nine in total, includingthe pre and post de-emphasis output stages, contribute a currentproportional to the total desired modulation current in order to definethe setpoint for the op-amp 310 controlling the LDO 305. For example,the current is 1/32 of the tail current for that particular stage. Thus,the modulation current setpoint is defined by a distributedarchitecture. Additional current for the bias is also summed in order toproperly drive the LDO 305 output voltage such that the desired powerlevel is achieved in the VCSEL 220. Due to the linear nature of thesumming of output main, pre, and post de-emphasis current paths, currentcan be summed into a termination load. In one implementation, thetermination load can be a 40 Ohms resistive element. The resistive valueof the termination load can be selected to allow for sufficient headroomfor a low dropout (LDO) regulator 225. In this manner, the range ofmodulation current settings, and a desirable high extinction ratio (ER),e.g., >6 dB, in the output optical signal may be achieved.

10021 The VCSEL 220 can be biased with a current Ibias. In oneimplementation, the bias current Ibias can be 6 mA. An exemplary zerolevel in VCSEL 220, e.g., representing data of 00 binary can be 2 mA. Anexemplary high level in VCSEL 220, e.g., representing data of 11 binarycan be 10 mA. Thus, four current levels correspond to the fourcombinations of two bits. For example, 2 mA corresponds to a minimumoutput power of a VCSEL in operation, e.g., a data value of 00 binary.

Im is a “tail current” through the differential driver pairs 315 and320, generated by current sources 325 and 330. A percentage “x” of thetotal Im is generated by current source 315, and the remainder of totalIm is generated by current source 320. Undesirable voltage ripple on theLDO 225 output is minimized by Relation 1, below, based on theresistance of VCSEL 220, referred to as “Rvcsel.”

x=Rvcsel/(Rvcsel+40)  (Relation 1)

The tail current Im, the total current for all exemplary nine stagescombined, is based on the resistance of VCSEL, referred to as “Rvcsel,”and the modulation current, Imod, which is equal to 48 mA (high levelcurrent minus level current), according to Relation 2, below:

Im=Imod*(Rvcsel+40)/40   (Relation 2)

Note that the total Im will include contributions from up to theexemplary nine stages, and will not necessarily be equally distributedamong the multiple stages. For example, a current contribution for anygiven stage may vary according to data and eye shaping requirements. Thecurrent sources 325, 330 can include differential data drivers, wherein“DinP” stands for Data In Positive, while “DinN” stands for Data InNegative.

Some current in every switching cycle bypasses LDO 305. If the totalcurrent passed through LDO 305, such total current would produce anundesirable ripple. Operational amplifier 310 controls the gate of LDO305 to generate the correct bias and modulation currents. The outputcurrent to VCSEL 220 goes through an RC low pass filter 335 as one inputto op amp 310. The other input to op amp 310 is a current that is aratio of the tail currents, which is a function of the bias currentdesired for the VCSEL plus the tail modulation. In some embodiments,some or all such current sources may be programmable.

As previously presented, it is necessary to know the resistance of VCSEL220 in order to set bias and modulation currents so as to minimizeripple. See, for example, Relation 1, above. Unfortunately, theresistance of different VCSELs varies, and is frequently not welldocumented by the VCSEL manufacturer. In addition, a VCSEL is oftenselected and/or provided by a party other than the manufacturer of anintegrated circuit device encoder/driver of FIG. 2 and/or currentfeedback loop of FIG. 3, and often selected after manufacture of such anintegrated circuit.

In accordance with aspects of the present technology, the resistance ofa VCSEL 220 may be determined in situ, for use in setting the tailcurrents of the differential driver pairs 315, 320.

In accordance with embodiments of the invention, the anode of VCSEL 220is coupled to an analog to digital converter 340 (not shown). Twodifferent currents are coupled through VCSEL 220. For example, a firstcalibration current of, for example, 4 mA, is coupled through VCSEL 220,and a first voltage measurement across VCSEL 220 is made via the analogto digital converter 340. A second calibration current of, for example,6 mA, is coupled through VCSEL 220, and a second voltage measurementacross VCSEL 220 is made via the analog to digital converter 340. Fromthe two voltage measurements with known currents, a resistance of VCSEL220 may be determined. In this novel manner, a resistance of a VCSELdoes not need to be known by the party selecting and/or operating aVCSEL for use in a transmitter system.

Referring now to FIG. 4, an encoder/driver and optical transmitter, inaccordance with aspects of the present technology, is shown. Theencoder/driver 400 can include a modulation current source 300 and aregulator 225, as described above. The encoder/driver 400 can furtherinclude a bias current generator 410, an analog-to-digital converter(ADC) 420, and control logic 430. The control logic 430 can be coupledto the bias current generator 410 and analog-to-digital converter (ADC)420. In one implementation, bias current generator 410 andanalog-to-digital converter (ADC) 420 can be coupled to the input of theoptical transmitter 220 as illustrated in FIG. 4. In anotherimplementation, the bias current generator 410 can be coupled to theinput of the optical transmitter 220 through the regulator 225 (notshown). In one implementation, the control logic 430 andanalog-to-digital converter (ADC) 420 can be implemented in a digitaldiagnostic monitoring information (DDMI) subsystem of the encoder/driver400. In one implementation, the control logic 430 can be implemented asfirmware by a microcontroller executing computing instructions stored inread only memory (ROM) of the encoder/driver 400. Alternatively, thecontrol logic 430 could be implemented by a processing unit executingcomputing instructions stored in memory such as but not limited torandom access memory (RAM), flash memory or the like. In yet anotherimplementation, the control logic 430 could be implemented as a finitestate machine (FSM). The bias current generator 410 and theanalog-to-digital convert (ADC) 420 can be coupled to the output of themodulation current source 300. Operation of the encoder/driver 400 willbe further explained with reference to FIG. 5, which shows a method ofconfiguring an encoder/driver for an individual unit of an opticaltransmitter.

The method of configuring the encoder/driver 400 can include receiving astartup event, at 510. In one implementation, the control logic 430 ofthe encoder/driver 400 can receive a startup event, such as but notlimited to a power up signal or reset signal. At 520, a first biascurrent can be applied to an input of an optical transmitter and acorresponding first voltage can be measured at the input of the opticaltransmitter. In addition, a second bias current can be applied to theinput of the optical transmitter and a corresponding second voltage canbe measured. In one implementation, the control logic 430 can cause thebias current generator 410 to apply a first bias current to the input ofthe VCSEL transmitter 220 in response to the input event. The controllogic 430 can cause the analog-to-digital converter 420 to measure thefirst voltage level at the input to the VCSEL transmitter 220 generatedby application of the first bias current. The control logic 430 can thencause the bias current generator 410 to apply a second bias current tothe input of the VCSEL transmitter 220 The control logic 430 can causethe ADC 420 to measure the second voltage level at the input to theVCSEL transmitter 200. For example, as illustrated in FIG. 6, the biascurrent generator 420 can apply a 4 milli Ampere (mA) current 610 to theinput of the VCSEL with the high-speed data path turned off, and the ADC420 can measure the resulting voltage. The bias current generator 420can then apply 6 mA current 620 to the input of the VCSEL and the ADC420 can measure the resulting voltage. In one implementation, the firstand second bias currents can be chosen to be above the turn on thresholdof the VCSEL, while at the time not being too high a current such thatthe input resistance of the VCSEL may become non-linear. The biascurrent can be trimmed at wafer probe during the manufacture of theencoder/driver 400 to a deviation of less then 1%. Furthermore, althoughthe bias current may vary slightly from unit to unit of theencoder/driver 400, the difference between the first and second biascurrent levels will vary equally within each unit. Therefore, the use ofthe difference between the first and second bias currents in calculatingthe input resistance value will further increase the accuracy of theinput resistance measurement.

At 530, an input resistance of the given individual unit of the opticaltransmitter can be determined from the difference between the first andsecond voltages divided by the difference between the first and secondbias currents. In one implementation, the control logic 430 cancalculate the input resistance of the particular optical transmitter 220by dividing the difference between the measured second voltage and themeasured first voltage by the difference between the second applied biascurrent and the first applied bias current. It is to be appreciated thatthe input resistance of the optical transmitter will vary to some extentfrom unit to unit due to manufacturing variances, operating conditionsand or the like. Accordingly, the input resistance of the individualoptical transmitter can be determined from the two different biascurrents applied to the input of the given optical transmitter and theresulting two different voltages. At 540, the modulation current outputby the encoder/driver 400 to drive the optical transmitter 220 can beconfigured based on the determined input resistance of the givenindividual unit. In one implementation, the control logic 430 canconfigure a modulation current of the modulation current source 300based on the determined input resistance of the individual VCSEL 220coupled to the encoder/driver 400. In one implementation, the modulationcurrent can be determined in accordance with Relation 3.

Im_set=Imod_ desired*(Rvcsel+40)/40  (Relation 3)

In one implementation, the modulation current can be configured on a perlane basis for all three non-return-to-zero lanes, top, mid and bottomin accordance with Relation 4.

Im_set(per lane)=Imod_desired*(Rvcsel+40)/120  (Relation 4)

It is to be appreciated that Im set can be utilized as Im in the currentsources 325 and 330 as described above with reference to FIG. 3. Theconfigured modulation current can be routed by the modulation currentsource 300 to the regulator 225 in order to reduce ripple voltage on theoutput of the regulator 225, thereby reducing vertical eye closure ofthe PAM-4 modulated signal to the VCSEL transmitter 220.

The determination of the input resistance of the VCSEL transmitter 220utilized for configuration of the modulation current of theencoder/driver 400 can advantageously reduce ripple voltage from theregulator 225 of the encoder/driver 400, which can in turn minimizevertical eye closure of the encoder/driver output signal. Thedetermination of the input resistance of the individual VCSELtransmitter 220 can also advantageously provide a measure of the healthof the VCSEL transmitter 220 and variation relative to a mean, which canbe a form of VCSEL process monitoring.

Referring now to FIG. 7, a method of configuring an encoder/driver foran individual unit of an optical transmitter, in accordance with aspectsof the present technology is shown. The method can include determiningan input resistance of the given individual optical transmitter unitcoupled to an encoder/driver, at 710. In one implementation, the inputresistance 440 of the particular optical transmitter 220 can bedetermined as described above with reference to FIGS. 4-6. At 720, auser desired modulation current for the optical transmitter canoptionally be received. In one implementation, a desired modulationcurrent 450 can be input by a user to the encoder/driver 400.

At 730, one or more configuration settings of a modulation currentsource of an encoder/driver can be set based on the received inputresistance of the optical transmitter and optionally the user desiredmodulation current. In one implementation, one or more settings can bestored by the control logic 430 in one or more registers 460 thatcontrol the modulation current generated by modulation current source300. At 740, the modulation current output by the encoder/driver fordriving the optical transmitter can be configured based on the one ormore configuration settings. In one implementation, the modulationcurrent source 300 can generate one or more modulation currents of thesum thereof for output to the optical transmitter 220 based on the stateof the one or more registers 460.

In contrast, in the conventional encoder/driver, the user (e.g.,customer) had to know the VCSEL input resistance from the componentmanufacturer's datasheet for the VCSEL. The user then needed tocalculate the required modulation current register values off-chip for adesired modulation current. Four different registers needed to be set bythe user on the PAM-4 encoder/driver chip with the user calculatedmodulation current register values. The conventional process incurreddata write time across a data interface such as inter-integrated circuit(I2C) communication link. The conventional process is also prone toerrors by the user.

Aspects of the present technology advantageously simplify the setting ofthe registers related to the modulation current for the VCSEL. Theconventional process of off-chip calculations based on the VCSEL inputresistance from the manufacturer datasheet is advantageously eliminated.The input resistance of the individual VCSEL can be advantageouslyautomatically determined by the encoder/driver in accordance withaspects of the present technology. The encoder/driver can alsoadvantageously automatically calculate the desired modulation currentfrom the on-chip measured VCSEL input resistance and optionally a userspecified desired modulation current. Furthermore, the correspondingregister values can be automatically set based on the desired modulationcurrent. Accordingly, aspects of the present technology advantageouslyprovide for configuration of the modulation current based on theindividual optical transmitter. Aspects also advantageously reduce thesetup time for configuring the modulation current of the encoder/driver.Aspects also advantageously reduce user error in the configuration ofthe modulation current of the encoder/driver. Aspects of the presenttechnology therefore can reduce the workload for the user and improveease of use.

The foregoing descriptions of specific embodiments of the presenttechnology have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit thepresent technology to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteaching. The embodiments were chosen and described in order to bestexplain the principles of the present technology and its practicalapplication, to thereby enable others skilled in the art to best utilizethe present technology and various embodiments with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto and their equivalents.

1. A driver comprising: a bias current generator; an analog-to-digitalconverter; and control logic configured to: control the bias currentgenerator to apply a first bias current to an input of an opticaltransmitter and control the analog-to-digital converter to measure acorresponding first voltage at the input of the optical transmitter;control the bias current generator to apply a second bias current to theinput of the optical transmitter and control the analog-to-digitalconverter to measure a corresponding second voltage at the input of theoptical transmitter; and determine an input resistance of the opticaltransmitter from a difference between the second and first voltagesdivided by a difference between the second and first bias currents. 2.The driver according to claim 1, wherein the control logic is configuredto control the bias current generator and analog-to-digital converterand determine the input resistance in response to a detected startupevent.
 3. The driver according to claim 1, wherein the first biascurrent and the second bias current are above a turn on threshold of theoptical transmitter.
 4. The driver according to claim 3, wherein thefirst bias current and the second bias current are below a level wherethe input resistance of the optical transmitter is non-linear.
 5. Thedriver according to claim 1, wherein the optical transmitter comprises avertical-cavity surface emitting laser.
 6. The driver according to claim1, wherein the driver is incorporated in a four-amplitude level of pulsemodulation encoder/driver.
 7. The driver according to claim 1, furthercomprising: a modulation current source; wherein the control logic isfurther configured to: set one or more configuration settings of themodulation current source based on the determined input resistance ofthe optical transmitter.
 8. The driver according to claim 7, wherein themodulation current source is configured to output a modulation currentto the optical transmitter based on the one or more configurationsettings.
 9. The driver according to claim 1, wherein the control logicis further configured to: receive a user desired modulation current; andset one or more configuration settings of the modulation current sourcebased on the determined input resistance of the optical transmitter andthe received user desired modulation current.
 10. The driver accordingto claim 9, wherein the modulation current source is configured tooutput a modulation current to the optical transmitter based on the oneor more configuration settings.
 11. A method of configuring anencoder/driver comprising: receiving a startup event; applying a firstbias current to an input of the optical transmitter and measuring acorresponding first voltage at the input of the optical transmitter inresponse to the received startup event, wherein the first bias currentis above a turn on threshold of the optical transmitter; applying asecond bias current to the input of the optical transmitter andmeasuring a corresponding second voltage at the input of the opticaltransmitter in response to the received startup event, wherein the firstbias current is above a turn on threshold of the optical transmitter;and determining an input resistance of the optical transmitter from thedifference between the first and second voltages divided by thedifference between the first and second bias currents in response to thereceived startup event.
 12. The method of claim 11, further comprising:configuring a modulation current output by the encoder/driver fordriving the optical transmitter based on the determined input resistanceof the optical transmitter.
 13. The method of claim 11, furthercomprising: setting one or more configuration settings of a modulationcurrent source of an encoder/driver based on the determined inputresistance of the optical transmitter in response to the receivedstartup event.
 14. The method of claim 11, further comprising: receivinga user desired modulation current for the optical transmitter; andsetting one or more configuration settings of a modulation current of anencoder/driver based on the determined input resistance of the opticaltransmitter and the user desired modulation current in response to thereceived startup event.
 15. The method of claim 11, wherein theencoder/driver comprises a four-amplitude level of pulse modulationencoder/driver.
 16. The method of claim 11, wherein the opticaltransmitter comprises a vertical-cavity surface-emitting laser.
 17. Amethod of configuring an encoder/driver comprising: determiningautomatically by the encoder/driver an input resistance of an individualoptical transmitter; and setting one or more configuration settings inone or more registers of a modulation current source automatically bythe encoder/driver based on the determined input resistance of theoptical transmitter.
 18. The method of claim 17, further comprising:receiving a user desired modulation current for the optical transmitter;and setting the one or more configuration settings of the modulationcurrent source in the one or more registers automatically by theencoder/driver further based on the user desired modulation current. 19.The method of claim 17, further comprising: configuring a modulationcurrent output by the encoder/driver for driving the optical transmitterbased on the one or more configuration settings in the one or moreregisters.